Re: [Q] Default SLAB allocator

From: Tim Bird
Date: Tue Oct 16 2012 - 14:40:14 EST

On 10/16/2012 11:27 AM, Ezequiel Garcia wrote:
> On Tue, Oct 16, 2012 at 3:07 PM, Tim Bird <tim.bird@xxxxxxxxxxx> wrote:
>> On 10/16/2012 05:56 AM, Eric Dumazet wrote:
>>> On Tue, 2012-10-16 at 09:35 -0300, Ezequiel Garcia wrote:
>>>> Now, returning to the fragmentation. The problem with SLAB is that
>>>> its smaller cache available for kmalloced objects is 32 bytes;
>>>> while SLUB allows 8, 16, 24 ...
>>>> Perhaps adding smaller caches to SLAB might make sense?
>>>> Is there any strong reason for NOT doing this?
>>> I would remove small kmalloc-XX caches, as sharing a cache line
>>> is sometime dangerous for performance, because of false sharing.
>>> They make sense only for very small hosts.
>> That's interesting...
>> It would be good to measure the performance/size tradeoff here.
>> I'm interested in very small systems, and it might be worth
>> the tradeoff, depending on how bad the performance is. Maybe
>> a new config option would be useful (I can hear the groans now... :-)
>> Ezequiel - do you have any measurements of how much memory
>> is wasted by 32-byte kmalloc allocations for smaller objects,
>> in the tests you've been doing?
> Yes, we have some numbers:
> Are they too informal? I can add some details...

> They've been measured on a **very** minimal setup, almost every option
> is stripped out, except from initramfs, sysfs, and trace.
> On this scenario, strings allocated for file names and directories
> created by sysfs
> are quite noticeable, being 4-16 bytes, and produce a lot of fragmentation from
> that 32 byte cache at SLAB.

The detail I'm interested in is the amount of wastage for a
"common" workload, for each of the SLxB systems. Are we talking a
few K, or 10's or 100's of K? It sounds like it's all from short strings.
Are there other things using the 32-byte kmalloc cache, that waste
a lot of memory (in aggregate) as well?

Does your tool indicate a specific callsite (or small set of callsites)
where these small allocations are made? It sounds like it's in the filesystem
and would be content-driven (by the length of filenames)?

This might be an issue particularly for cameras, where all the generated
filenames are 8.3 (and will be for the foreseeable future)

> Is an option to enable small caches on SLUB and SLAB worth it?
I'll have to do some measurements to see. I'm guessing the option
itself would be pretty trivial to implement?
-- Tim

Tim Bird
Architecture Group Chair, CE Workgroup of the Linux Foundation
Senior Staff Engineer, Sony Network Entertainment

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